Product categories allow you to associate multiple products for use within aMember. As shown in the 'Product Categories' screen (see below) the categories mainly serve to manage a hierarchy of products for display purposes.
aMember does not respect category hierarchy. Each category is absolutely independent. You can use hierarchy only to organize your categories.
However, Product Categories also are used by 'Protect Content' options in the 'Access Permissions' setting so that a content item may be marked as being available to a Product Category and each product that you have associated with the Product Category will have that content available.